Field of the Invention
The present invention relates to a phase-shifting multiplication type FM signal demodulation circuit.
FIG. 1 is a circuit diagram showing a demodulation circuit of the above type. In the figure, reference numeral 1 denotes an input terminal for FM signal S.sub.1, 2 a local oscillation circuit, 3 a frequency mixer which functions to mix the FM signal S.sub.1 from the input terminal 1 with a signal S.sub.3 output from the local oscillator 2 thereby to produce an FM intermediate frequency signal S.sub.2 (10.7 MHz), and numeral 4 designates an amplifier for amplifying the intermediate frequency signal from the frequency mixer or converter 3. The amplifier 4 may be composed of a direct-coupling type differential amplifier having the function of limiting the amplitude of the intermediate frequency signal. The phase-shifting multiplication type demodulation circuit for demodulating the signal from the amplifier 4 is denoted generally by reference numeral 5 and includes a first differential switching circuit 120 A composed of paired transistors 121 and 122 and adapted to be switched by the FM intermediate signal S.sub.2 from the amplifier 4, a phase shifting element 220 designed to shift the phase of the FM intermediate signal, a phase-shifter apparatus 200 comprising a resonance circuit 210 for converting the phase or frequency of the FM intermediate signal S.sub.2 ' which has been phase-shifted by the phase shifting element 220, a second differential switching circuit 120B composed of paired transistors 123 and 124 and adapted to be switched by the phase-converted signal available from the phase-shifter circuit or device 200, a detector circuit 170 for detecting the signal appearing at the output stage of the second switching circuit, and an output resistance 300' for determining the magnitude of the demodulated output from the demodulation circuit. The major portion 100 of the FM demodulation circuit 5 as represented by broken lines is implemented in a form of an integrated circuit configuration. The resonance circuit 210 has a resonance resistor 213 which is provided as an externally accessible element so that the distortion of the demodulated output may be reduced. Numeral 6 denotes a low-pass filter and 7 a tuning indicator.
In the demodulation circuit shown in FIG. 1, the first switching circuit 120A is driven by the FM intermediate frequency signal S.sub.2 supplied to the input terminal 100 a of the demodulation circuit, while the second switching circuit 120B is adapted to be driven by the signal S.sub.2 ' phase-shifted through the phase shifter circuit 200. Accordingly, in the case where a constant current I.sub.130 from a constant-current supply source is allowed to flow through diodes (or resistors) 161 and 162 provided at the output of the second switching circuit 120B only for a period during which an input signal is present at the input of the second switching circuit 120B, and diodes (or resistors) 161 and 162 provided at the output stage of the second switching circuit 120B, a current signal having a pulse width corresponding to the phase difference between the signals S.sub.2 and S.sub.2 ' will flow through the diodes 161 and 162 and give rise to variation in the voltage across the diodes 161 and 162. The averaged value of such voltage variation is derived through a low-pass filter 400. In this manner, D.C. voltage variation corresponding to the phase difference described above can be obtained and thus an FM modulated input signal will be demodulated. Since the demodulating operation of the demodulation circuit 5 is based on the same principle as that of the demodulation circuit described hereinafter, detailed description of the demodulating operation is omitted at this point.
In connection with the demodulation circuit unit of the type described above, it is frequently demanded by the users that the magnitude of the demodulated output from the demodulation circuit could be conveniently or optionally changed in dependence upon the variety of amplifiers or the like circuits connected as a succeeding stage of the demodulation circuit. However, it is difficult to satisfy such demand in practice in the case of the demodulation circuits for the following reasons.
Namely, in the case of the demodulation circuit shown in FIG. 1, two means are conceivable for varying the magnitude of the demodulated output. The first means will be the externally accessible resonance resistor 213 of the resonance circuit 210 which can be so adjusted as to influence the distortion as well as the magnitude of the demodulated output. Another means may be provided by an output resistor 300' which is also provided as an externally accessible element at the time of manufacturing the major portion 100 of the demodulation circuit 5 in an integrated circuit so that the resistance value of the discrete output resistor 300' may be externally varied.
The first means is however disadvantageous in that distortion of the demodulated output will undergo changes as the output is varied. More particularly, when the resistance value R.sub.213 of the resonance resistor 213 is increased, then the frequency-versus-phase characteristic (FIG. 4) of the resonance circuit 210 will be changed with the inclination or slope of the characteristic curve being increased or steepened, which brings about an increase in the distortion along with the increased magnitude of the demodulated output. On the other hand, decreased resistance value R.sub.213 of the resonance resistor 213 will result in a small slope of the above characteristic curve, involving decrease in the magnitude and distortion of the demodulated output. Such phenomenon is generally known in the art. Further, the switching function of the second switching circuit 120B constituting the gate circuit 120 will tend to become imperfect. This is because the resonance resistor 213 operates also as the bias resistor for the second switching circuit 120B and decreased resistance value of the resistor 213 is inevitably accompanied with a decreased bias voltage applied to the second switching circuit 120B.
The second means described above is impractical in that the output resistor 300' can not be implemented as a discrete externally accessible element, because in such cases the demodulated output appearing at the output terminal will become non-uniform for the following reasons.
In general, in the case of the integrated circuits, deviation of resistance values of resistors can be retained in the range of .+-.5%. However, the deviation of resistance in the absolute value would amount to .+-.30%. By way of example, assuming that the individual resistors of the integrated circuit 100 are deviated from the desired or normal values for +30% and the resistance values become 1.3 times as great as the latter, the constant current I.sub.130 flowing in the constant-current source circuit 130 will be decreased to 1/1.3 as accompanied with correspondingly decreased current flow in various circuitries. The current flowing through the output resistor 300' will be of course decreased. Assuming again that the output resistor 300' has resistance value deviated for +30% as is in the case of the individual resistors of the integrated circuit 100 and has 1.3 times as great as the desired value, the voltage drop appearing across the output resistor 300', that is, the output signal of the demodulation circuit may remain uniform, so long as the resistor 300' is incorporated in the integrated circuit. However, if the output resistor 300' is provided as an external discrete element with a view to allowing the adjustment of the demodulated output, there will appear non-uniform or fluctuating signals at the output terminal of the demodulation circuit because the deviation of the resistance value of the discrete output resistor 300' will differ from those of the integrated resistors in addition to the fact that there is no correlation between the magnitude of the constant current I.sub.130 determined by the internal resistances of the integrated circuit and the deviation of the resistance value of the external output resistor 300'. For these reasons, the output resistor 300' could not be provided as the external discrete element in this demodulation circuit.